1. Field of the Invention
The present invention relates to a process detection apparatus for use in a chemical plant or the like to control fluids such as liquids and gases, which measures the pressure of a fluid, the pressure difference of fluids and the level of a fluid in a tank and is used to effect control of operation of the whole plant, and more particularly, it relates to a compact, light-weight, process detection apparatus for which it is to perform maintenance. In a process detection apparatus which outputs a signal representing the measured state of a fluid to an external equipment and has an indicator provided therein to also display the state of the fluid on the indicator, the present invention relates to an improvement that a signal to be transmitted to outside and a signal to be displayed on the indicator are converted according to a predetermined function and outputted after the conversion and the indicator indicates that these data are the ones already converted.
2. Description of Related Art
It is well known that certain operation parameters such as measuring pressure, belt speed, and flow rate in a chemical plant is indispensable to operating the plant in a safe and efficient manner. To this end, differential pressure/pressure measuring instruments have been used as a process detection apparatus. In pressure differential measurements, flow rate, density and level can be measured, while in pressure measurements, in addition to positive and negative pressures, a liquid level in a container such as a tank can also be measured. Since such differential pressure/pressure measuring instruments for the above measurements are frequently used in an explosive atmosphere which contains combustible gas or vapor of flammable material, the instruments shall be type-accepted as explosion-proof according to standards for explosion-proof electromechanical equipment. As shown in a pressure differential/pressure transmitter which Japanese Patent Unexamined Publication No. 4-5532 has disclosed, such a differential pressure/pressure measuring instrument comprises a pressure receiving section for sensing pressure differential and pressure, and a signal converter for converting a signal from the pressure receiving section into an analog signal or a digital signal. A connection between the pressure receiving section and the signal converter conforms to the shape and dimensions specified in standards to assure explosion-proofness entirely.
FIG. 2 shows a typical construction of a conventional pressure measuring instrument. In the figure, a pressure receiving section or pressure sensing block 202 has a pressure sensor or a differential pressure sensor mounted therein. A flange 204 having a pressure input hole 203 for applying a pressure from outside to the sensor is attached through a gasket to the pressure or detector block 202 by screws 205. A tubular connection or junction block 207 is welded to the detector block 202 to transfer signals from the pressure sensing block to a signal converter. A connector and the like to which sensor signals are provided via lead wires are attached on the junction block 207.
A signal converter case 210, which is typically made of cast aluminum, is secured to the junction block 207 by screws 212. The converter case 210 houses an electronic circuit which includes electronic parts for driving the sensors, operationally amplifying sensor signals and converting them into analog signals or digital signals, and an indicator 215 for indicating output data. A cover 216 is threadedly attached to the converter case 210, and the cover has a glass window through which the indicator indicating the analog data can be seen.
The signal converter case 210 typically includes an amplifier block 220 for processing the signals from the detector block and a terminal board box 222 housing a terminal board for receiving power supply from outside and for delivering the power to the amplifier. The analog or digital indicator built in the amplifier block 220 inside the converter case 210 allows the output signal from the detector block to be monitored.
Output signals from the instrument are provided to external high-level control equipment via a cable connected to the instrument through a cable entry thereof.
As described above, a conventional process detection apparatus is essentially made up of three components of the pressure sensing block, the amplifier block and the terminal board box, the latter two of which are included in the signal converter case, and therefore, the entire apparatus is large in size and its construction is complicated. When it is desired that the detector instrument meets with pressure resistance and explosion-proof requirements, its design becomes larger and more complex, thereby having unfavorable influence on its handling. Further, miniaturization has rapidly advanced in electronic components today. Accordingly, there is a need for a compact detector which can be mounted on instrumentation piping in a direct line mount manner so that the operation and maintenance of a plant is carried out in a labor-saving manner.
When a line mount method is employed in an effort to reduce the installation cost at the stage of planning a new plant, detectors themselves must have a structure capable of sufficiently withstanding vibrations and shocks encountered on piping. Because of the large and complicated design, the conventional detectors can not bear such vibrations. A compact design for the detectors not only means less installation space but also contributes to saving of energy and material resources. Further, it is desired that detectors have a structure which facilitates their maintenance.
In commercial plants, the flow rate of a fluid flowing through a pipeline has been conventionally determined as follows. A pressure differential .DELTA.P generated by an orifice disposed in the pipeline is detected by a process detection apparatus. The detector outputs a signal which is received by a signal converter via a two-wire transmission line. The signal converter converts the signal according to a certain function, and transmits the converted signal to high-level control equipment which finally calculates or determines the flow rate. There is a need for a process detection apparatus which has the signal conversion function for flow rate determination that has conventionally been performed by the high-level control equipment, as well as a data indicator function that can tell an operator that what indicated on and outputted from the detector are the data that have already been converted.
Furthermore, there is a need for a process detection apparatus which is provided with an indicator having a function of telling an operator its zero point adjustment state so that the operator can easily perform zero point adjustment on the occasion of maintenance.